Mazuma Mobile

Mazuma is straight forward, fast, and easy to use

Really good, totally trustworthy, they pay fast and the whole process is very easy. Type in the model of your phone, get a quote, chose how you want to be paid and apply for their free-post envelope. It arrives quickly and once you've sent your phone off the money appears in your account within a couple of days.
I've used them a couple of times now and have always been very happy.